This invention relates to improvements in a recording system for recording information on a record medium by using an energy beam.
When a beam of light or the like is used to record and reproduce information on and from a record medium such as a tape or a disc, the phase information in a signal reproduced from the record medium must be made to coincide with the phase information in a recording signal. For example, when a frequency modulated video signal is recorded and reproduced on and from a record medium, a reproduced frequency modulated wave may have its symmetry destroyed although a corresponding recorded frequency modulated wave had good symmetry. Under these circumstances, a reproduced wave includes a second harmonic which, in turn, spectrally overlaps a side band of the fundamental wave. As a result, the FM demodulation causes, in addition to the video signal, beat interferences. Furthermore, upon the recording and reproducing of PCM digital signals, the digital demodulation thereof may be impossible if phase information is not well preserved between the recording and reproducing. Why the phase information is not well preserved between the recording and reproducing will now be described, by way of example, in conjunction with the recording of signals with a beam of light. Photosensitive materials used with such recording are generally difficult to develop a halftone thereon so that their portions which are sensitive to light can be, in many cases, definitely distinguished from their portions which are insensitive to light at some light level. Accordingly, when such a photosensitive material varies in its photosensitive level during a recording process, the same optical signal forms different record traces. For example, when the photosensitive material has its photosensitive level lying just on the average level of a waveform for the optical signal, the resulting record trace has a duty cycle of 50% and does not include a second harmonic. However, when the photosensitive level does not coincide with the average level of the waveform for the optical signal, the duty cycle of the resulting record trace is not of 50% and a second harmonic is caused, resulting in a reproduced signal which is deteriorated in its quality. In order to decrease the second harmonic of the reproduced signal, there has been previously proposed a method of strictly controlling the intensity of the light beam and the photosensitive level of the photosensitive material to constant magnitudes respectively. This method has been extremely difficult to be actually carried out because of changes in parameters concerning the light beam and photosensitive material.
Accordingly, it is an object of the present invention to provide a new and improved recording system for accurately preserving phase information upon recording, not only for frequency modulated signals but also for signals varied in their duty cycle, and for recording without the influences of both the instability of the source of the energy beam involved and the uneveness of the sensitivity of the record medium involved.